(1) Field of the Invention
This invention relates to a process for the production of bobbins suitable for use in voice coils of speakers.
(2) Description of the Related Art
FIGS. 7(a) through 7(c) illustrate a usual process for the manufacture of voice coils. First of all, a bobbin 12 is fit on the outer peripheral wall of a cylindrical jig 11 as shown in FIG. 7(a). The bobbin 12 is generally made of paper, but an aluminum foil or the like may also be used in some instances. The cylindrical jig 11 is used for the maintenance of the roundness of a voice coil and its positioning upon its manufacture and its assembly in a speaker. The cylindrical jig 11 is therefore pulled out after the assembly of the voice coil in the speaker. A coated wire is next wound in layers without spacing between adjacent turns and between adjacent layers on the bobbin 12 as depicted in FIG. 7(b), so that a coil 13 is formed and leads 14 of the coil 13 are caused to extend in the direction of the longitudinal axis of the bobbin 12. Thereafter, a holding tape (or an adhesive) 15 is caused adhere on the bobbin 12 in such a way that the holding tape 15 holds the lead 14 in adjacent to the coil 13 as illustrated in FIG. 7(c). A voice coil 16 in a form fitted on the cylindrical jig 11 is completed as described above.
Incidentally, the outer peripheral wall of the bobbin 12 has been coated in advance with an adhesive which is soluble in an alcohol. The coated wire, which has been coated beforehand with an alcoholic solvent and is self-bonding, is wound on the outer peripheral wall of the bobbin 12. When they are heated and dried, the bobbin 12 and coil 13 are unified.
FIG. 8 schematically illustrates a conventional process for the production of bobbins suitable for use in such voice coils. Two bobbin tapes 20a,20b are used. One of the tapes, namely, the inner tape 20a is helically wound with its both side edges maintained in abutment, and the other tape, namely, the outer tape 20b is also helically wound on the tape 20a in such a way that the abutment between both side edges of the tape 20b is displaced by half a pitch from that of the tape 20a. The tapes 20a,20b are bonded together with an adhesive (not shown) to form a double-walled tube 21, which is then cut into a prescribed length so as to obtain the bobbin 12.
A bobbin produced in such a manner as described manner tends to develop air bubbles in the adhesive layer between the inner and outer tapes 20a,20b upon drying, so that its quality is instable. Even if such bubbles cause no problem at the stage of production of a voice coil, there is a potential problem that after assembled in a moving-coil speaker, the bubbles may expand under heat to deform the voice coil, the clearance between the voice coil and the associated pole plates of the moving-coil speaker may hence vary or the voice coil may thus contact either one or both of the associated pole plates, and the performance of the moving coil speaker may be deteriorated accordingly. There is another potential problem that since the double-walled tube formed by helically winding the tapes is cut at a right angle relative to the axis of the tube, the side edges of each of the tapes 20a,20b both extends at an acute angle relative to each of the thus-cut ends of the bobbin and the tapes 20a,20b are hence prone to peeling at the ends of the bobbin.
With a view toward solving such problems or in the case of a tape hard to be formed into such a helically-wound double-walled tube as in FIG. 8, it is also practised to form the bobbin 12 by cutting a paper tape into a short strip, rolling the short paper strip (which is designated at numeral 22 in FIG. 9) around an unillustrated cylindrical jig with both side edges 24 of the short strip opposing each other as depicted in FIG. 9 and then tentatively holding the opposing side edges 24 with an adhesive tape 25, a rubber band or the like at an area not extending to a coil-winding portion 23.
Since the short paper strip is rolled into the cylindrical shape in the above process, there is a potential problem that an axial displacement may often occur between the opposing side edges 24 as shown in FIG. 10 and the short paper strip 22 may thus be peeled off at corners thereof due to the displacement to result in poor performance. Although it is also practised to use a round-cornered paper strip 22 as shown in FIG. 11 with a view toward avoiding such a problem, this approach is accompanied by a further problem that the use of such a round-cornered paper strip unavoidably leads to a higher manufacturing cost because it must be formed by die punching.